1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an optical alignment composition containing polyimide having excellent thermal stability, an alignment layer formed using the optical alignment composition, and an LCD having the alignment layer. 2. Description of the Related Art
In general, as shown in FIG. 1, an LCD has a pair of upper and lower substrates 2 and 2' spaced apart and opposed to each other. Transparent electrode layers 3 and 3' are respectively formed on the upper and lower substrates 2 and 2'. Insulating layers 4 and 4' and alignment layers 5 and 5' are sequentially formed on the transparent electrodes 3 and 3'. A liquid crystal layer 7 is formed in the space between the upper and lower substrates 2 and 2'. Polarizing plates 1 and 1' for polarizing entering and transmitting lights are provided on the exterior of the substrates 2 and 2', respectively.
In the LCD having the aforementioned structure, according to an externally applied voltage, the arrangement of liquid crystals influenced by the electric field is changed. According to the changed arrangement, external light introduced to the LCD is shielded or transmitted. The LCD is driven by such a property. In other words, if a voltage is applied to the transparent electrode layers 3 and 3', an electric field is formed in the liquid crystal layer 7. Thus, liquid crystals are driven in a predetermined direction. The light introduced into the liquid crystals of the LCD is shielded or transmitted according to the driving direction of the liquid crystals.
The functions of the LCD as a display device, i.e., light transmittance, response time, view angle or contrast, are determined by the arrangement characteristic of the liquid crystal molecules. Therefore, the technology of controlling the uniform alignment of the liquid crystal molecules is very important.
The uniform alignment state of the liquid crystals is difficult to accomplish by merely interposing the liquid crystals between the upper and lower substrates. Thus, as shown in FIG. 1, it is know to form the alignment layers 5 and 5' for aligning liquid crystals on the transparent electrode layers 3 and 3'.
The alignment layer is conventionally formed by a rubbing method in which a thin film made of an organic polymer material such as polyimide or polyamide is formed, cured and then rubbed with a special cloth.
The rubbing method is easy to conduct and the process thereof is simple. However, minute particles or materials such as cellulose may separate from the cloth used in the rubbing treatment and contaminate the alignment layer. Further, depending on the material for forming the alignment layer, the alignment may not be accomplished smoothly. A thin film transistor may be damaged by static electricity generated during the rubbing treatment.
To solve the above-described problems, an optical alignment technology has been developed in which dust, static electricity or other pollutant particles are not generated and cleanliness is maintained during the overall process. According to such a non-destructive alignment method, polarized light is irradiated onto the optical alignment layer to cause anisotropic photopolymerization. As a result, the optical alignment layer has and alignment characteristic, thereby uniformly aligning the liquid crystals. The polymer for the optical alignment layer includes polyvinylcinnamate (PVCN) and polyvinylmethoxyxinnamate (PVMC). However, although they have an excellent optical alignment property, such polymers have poor thermal stability. In other words, the thermal stability of an alignment layer depends upon that of the polymer, which depends upon a glass transition point and cross linking density. Since the PVCN or PVMC has a relative glass transition point of 100.degree. C. or below, post-alignment thermal stability is decreased.